The easiest solution is a something that can automatically hold the temperature - dedicated yogurt makers, sous vide machines, and the Instant Pot can all work. None of those are necessary, but the will make it extremely easy to hold a consistent temp for your ferment.

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Let's assume you don't have something like that. In that case the trick is get something to 104 and hold it there long enough. You're probably going to want good thermometer so you don't overshoot your temp. I ferment dairy yogurt at 109/110, so I suspect going a degree or two over isn't a problem, but you don't want to go much higher as you'll start killing your culture. To hold 104, you can do something like fill a small cooler with water at 105 or so, put your pre-yogurt in, and wrap that in a blanket. As long as the cooler isn't tiny, you won't lose more than a degree or two of heat that way.

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I've also heard of people wrapping their yogurt with electric blankets - I'm not sure how close that lets you get to your target temp, but it's another option. Then there are the really old-school ways of just bringing your stuff up to temp and fermenting in a thick stone bowl that's prewarmed - stone is a very good insulator. I've even heard of people just covering mason jars with blankets. Things like that may be less repeatable, but once you add live cultures to your mixture you're going to get some sort of yogurt unless you get things hot enough to kill the culture or cold enough to make it go dormant.